Mass spectral studies of naphthoflavones

The electron impact induced fragmentations of the five possible naphthoflavones have been studied with the aid of low‐ and high‐resolution measurements, metastable decompositions and isotope labelling using either deuterium or carbon‐13 atoms. All compounds show both the direct expulsion of a CO residue from the carbonyl group and the retro Diels–Alder cleavage. The abundant [M–H]+ ion has been formulated as the substituted β‐tropolone species, which, in the case of β‐naphthoflavone, is in competition with a pentacyclic species arising from the elimination of H‐5 as evidenced in the spectrum of 5‐d‐β‐naphthoflavone. Other previously unsuspected ions are also in evidence.